1. Field of the Invention
This invention relates to high frequency power supplies and more particularly to a circuit for use in such supplies for providing frequency and phase synchronization therebetween when two or more identical such supplies are connected in parallel across a load.
2. Description of the Prior Art
Many types of power supplies now make use of the well-known pulse width modulation (p.w.m.) technique to provide a signal to drive one or more power switching devices at a relatively high frequency. The power supply may be of the type wherein the p.w.m. signal is used directly to drive the one or more power switching devices or it may be of the type wherein the p.w.m. signal is used to derive signals such as phase shifted squarewaves to drive the power switching devices.
The p.w.m. technique makes use of a sawtooth waveform of predetermined frequency for purposes of generating the drive signal to the power switching devices. Typically the frequency of the sawtooth waveform is in the order of 20 KHz when the supply has a single power switching device and in the order of 40 KHz when the supply has two or more power switching devices arranged, for example, in a half bridge or full bridge configuration.
In many such high frequency supplies an oscillator is included to generate pulses which are usually at the same frequency as the desired sawtooth waveform. The occurrence of these pulses is then used by appropriately arranged circuit elements as a signal for terminating the prior cycle of the sawtooth waveform and initiating the generation of the next cycle thereof.
In many instances the specifications of the system wherein a high frequency power supply of the type described above is to be used require that two or more identical such supplies be connected in parallel across the system load. The supplies may share the load on a substantially equal basis or one or more supplies may provide power to the load while the remaining paralleled supplies serve as back-up supplies which also provide a small amount of power to the load. In any case, it is desirable that the paralleled supplies operation be synchronized with each other, both in frequency and phase.
Such synchronization of operation could theoretically be obtained by adjusting the frequency of the oscillators of each supply such that all of the paralleled supplies generate their high frequency pulses at the same time. All of the paralleled supplies would then terminate one cycle of the sawtooth waveform and initiate generation of the next cycle at the same time. In this manner, the operation of each of the paralleled supplies would then be synchronized with respect to each other.
While such an adjustment would theoretically provide the desired synchronization, it is not possible on a practical basis to adjust the oscillators of each supply in such a manner. In fact, even if it were possible to so adjust the oscillators prior to the shipment of the supplies from the factory, the variation in oscillation component values as a result of usage would cause the paralleled supplies to lose their synchronization with each other in a relatively short amount of time. In addition, any necessity to change oscillator components in the field and still have synchronism would present the serviceman with a task beyond that ordinarily associated with such endeavors.
Another way in which paralleled supplies can be synchronized in operation is by the use of a master clock signal which is supplied from a source external to the paralleled oscillators. While such a source might be readily available where the load is of the type which includes its own clock source, there are many types of loads which do not have their own clock. In addition, while such a clock signal may be obtained from an external source which is not in the load, operation of the supplies in parallel then requires that they each be connected to that source. Finally, the use of such a clock makes synchronization of the paralleled supplies dependent on a source whose loss will surely cause the supplies to operate in a non-synchronous manner.
It is, therefore, desirable that synchronism between paralleled identical high frequency supplies be obtained in a manner which does not require careful initial and possible continual adjustment of the frequency of the oscillators used therein. It is also desirable that synchronism be obtained without reliance on an external source of master clock signals. It is also desirable that at any given time the supply whose oscillator runs at the highest frequency be used as the master and that the oscillators of the slower running supplies be synchronized thereto. It is also desirable that each of such paralleled supplies have circuitry included therein which allows this synchronization to take place in a relatively simple and inexpensive manner. Finally, it is desirable that this circuitry have sufficient flexibility such that as oscillator operating frequencies change with changes in oscillator component values the choice of the supply used as the master shift to that supply whose oscillator operates at the highest frequency. Such circuitry ensures that slight variations in oscillator operating frequency between identical paralleled supplies does not prevent the supplies from being synchronized in phase and frequency with each other. Such variations may arise as described above from the inability to adjust all supplies to the exact same operating frequency at the factory as well as from the change in component values with age.